This invention relates to an improved method of casting, and more particularly, directional solidification where the rate of withdrawal is purposefully varied, together with articles made by the improved process.
Directional solidification is a process for producing metal articles with aligned structures and resultant advantageous mechanical properties. The apparatus and methods for forming directional solidified articles are well documented in the literature. The most preferred from a commercial standpoint is the withdrawal technique in which a mold, after pouring, is vertically withdrawn from within a furnace hot zone into a cold zone at a rate which causes a solidification interface to move uniformly upward within the casting along its principal stress axis. Past improvements in the withdrawal technique have related to control of the hot zone temperatures, the spatial disposition of the chill plate means, baffling between the hot zone and the cold zone, and increasing the rate of heat loss which occurs in the cold zone. In the past it has been the practice to withdraw castings from the hot zone at a uniform rate, or stationary periods in alternation with a uniform rate as described in Giamei et al, U.S. Pat. No. 3,700,023.
The optimum rate of withdrawal is a function of the configuration of the apparatus, the configuration and composition of the mold, the solidification characteristics of the metal alloy, the microstructure sought in the casting, and economic considerations. One of the most significant relationships is that between the temperature gradient, G, and the solidification rate, R. Generally, it is found with most eutectic superalloys to which directional solidification is applied that the G/R ratio must exceed a characteristic value for each alloy; this critical value is designated (G/R)*. Most commonly, this value is equal to the ratio .DELTA.T/D, where .DELTA.T represents the alloy melting range and D represents the effective diffusion coefficient of solute atoms in the liquid metal. It follows that, to maximize the permissible solidification rate and thereby increase production and minimize metalmold reaction, it is desirable to increase the temperature gradient G in the molten metal. An advantageous method for doing this is that described in Tschinkel et al U.S. Pat. No. 3,763,926 and Tschinkel et al U.S. Pat. No. 3,915,761, both of the present assignee, where the mold is withdrawn to be immersed in molten metal as it passes into the cold zone; this technique is called Liquid Metal Cooling, or LMC.
Experience has shown that to obtain preferred microstructures it is desirable to maintain the solidification interface as essentially flat, that is, perpendicular to the direction of solidification. The reason for this follows: for a fixed temperature gradient, G, if R is too high or too low the solidification interface will be curved convexly upward and downward, respectively. This non-unidirectional condition will be indicative that growth on a microscopic basis will not be aligned with the principal axis along which the casting is being solidified. Therefore, depending on the practical deviation from unidirectional growth the casting will have deviant crystallographic or grain growth and resultant inferior properties. Furthermore, if a casting is solidified too fast extraneous nucleation may occur ahead of the solidification interface, disrupting the desired uniform directional or crystallographic nature of the solidification. On the other hand, if the solidification rate is too slow there is a likelihood of dendritic remelting and associated defects, mold interactions which degrade the alloy, and unduly long and uneconomic process times. The interrelationships between the aforementioned solidification parameters are extensively described in the paper "Computer Applications in Directional Solidification Processing" by Giamei and Erickson in the Proceedings of the Third International AIME Symposium on Superalloys, at Seven Springs, Pennsylvania, Sept. 19, 1976, pp 405-424; B. Kear, Ed., Claitor, Baton Rouge, La, 1977., which is hereby incorporated by reference.
Experience has shown that usually the conditions of optimum solidification are those where there is a maximum withdrawal rate without nucleation. These conditions establish the optimum economic situation. Consequently it is an objective of solidification process improvements to achieve these conditions in a casting. And when a mold has a constant cross-section and is in an apparatus generating a particular temperature gradient with a particular alloy, the maximum withdrawal rate can be established and achieved. However, most commercial products have instead varying cross-sections; a good example is a gas turbine airfoil which has a heavy root section, a thin and usually tapered airfoil section and often a heavier shroud at the airfoil tip. For such items the past practice has been to choose a fixed rate which will satisfactorily solidify the heavy and thin sections as the solidification interface passes through both of them. This of course requires some compromise in the optimization of the parameters mentioned above and results in a compromise in the structure which is achievable in the sections having different sizes. Also since the maximum rate is set by the heavier section from which the heat extraction is most difficult and where the thermal gradient is most difficult to make steep, the rate of production is accordingly limited.
Directional solidification can be used to make materials for any composition for any application, but it most particularly has recently been used to make three increasingly common but still quite advanced categories of materials for commercial gas turbine use: those which have columnar grain, single crystal or aligned eutectics. Many commercial superalloys such as U-700, B-1900, MARM-200 and IN-100 have been converted into columnar grain and single crystal form; other specialized ones have been developed for these unique structures, among them hafnium-modified MARM-200 and B-1900 and the alloys referenced in Example 3 herein. Eutectic alloys of commercial interest are typically multi-element alloys with very narrow temperature differentials between the liquidus and solidus (typically 5.degree.-20.degree. C.). They are often difficult to consistently solidify in a directional manner, because the parameter .DELTA.T/D and consequently (G/R)* is high (on the order of 125.degree. C.-hr/cm.sup.2), and even with high gradients, the maximum R is a relatively low speed (&lt;1 in/hr) and easily exceeded without proper control.
With a given apparatus-produced thermal gradient, the effect of R on the microstructure of the foregoing types of alloys can be summarized as follows: for low values of R in nickel base alloys a planar microstructure will result; as it is increased a cellular and then a dendritic structure will result. With these changes improved tensile and creep properties may result. In eutectic alloys the trend is similar except that when very low growth rate values are encountered, a coarse lamellar or "trapezoidal" structure can be formed; as the values are increased the structure becomes finer, then cellular, then dendritic.
The need for carefully controlling the thermal conditions and directional solidification rate has been long recognized. For example, Kraft U.S. Pat. No. 3,124,452 of the present assignee, described methods for unidirectionally forming eutectic alloys with spaced lamellae by regulating the solidification rate and thermal gradient and controlling the ratio between them within a broad range of 0.1.degree. to 1,000.degree. C.-hour per square centimeter. The desirability of high thermal gradients and the higher solidification rates that may be associated therewith is taught by Tien et al U.S. Pat. No. 3,677,835, of the present assignee. Tien also teaches suppression of the mushy zone in front of the solidification interface in order to inhibit dendritic growth and provide cellular and plane front structures in nickel alloys, particularly high gamma prime alloys such as NX188. Low solidification rates of the order of 0.2 inches per hour are mentioned. Smashey U.S. Pat. No. 3,897,815 discloses the application of hot zone heat at a plurality of rates to control the molten state of the casting and the directionality of solidification when the mold is being withdrawn from the hot zone. Coordination of the withdrawal and heating is mentioned as the general relation G/R suggests.
It has occasionally been the practice to remain stationary or go slowly after the metal is first poured into the mold, to allow the metal to stabilize within the mold, and to achieve good initial grain or crystal growth from the chill plate or single crystal starter, as the case may be. While the interrelation and importance of the apparatus-effected parameters and the material-effected parameters on the final casting has been recognized, the production of complexly shaped casting parts has not heretofore been generally conducted with other than a compromise unitary withdrawal rate or possibly a single change in growth rate. This may be attributed to the path and state of development of directional solidification. Processes originated in the laboratory for simple constant-cross section samples where unitary rates are entirely appropriate. They were largely transferred in toto to the still-infant production and preproduction of complex shapes. Now, there is more economic incentive to optimize procedures. Also, the criticality of microstructure has become better recognized as designers become more adept at using the new materials near their limits, producing the need for more careful control of solidification. Another factor has been the use of different alloys which have much more sensitivity of properties-to-microstructure and microstructure-to-solidification rate, than the earlier alloys.
The manufacture of eutectic alloy turbine blades which have varied structures, e.g., aligned airfoil with equiaxed root, as in Endres et al U.S. Pat. No. 3,790,303, has been previously disclosed. The solidification conditions were varied, but only to the extent necessary to achieve these variant structures within the same part, and problems of correlating solidification conditions with section size changes apparently did not arise.
Finally, whatever limited recognition there may have been of the appropriateness of changing growth and withdrawal rates from one casting section to another, no prior record indicates recognition of a systematic way of choosing rates for one section with respect to another, nor of the need or manner of transitioning controllably between different rates.